Cleveland-Cliffs Inc. Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Nonemployee Directors) (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	1,285,419
Share-based Payment Arrangement, Nonemployee [Member] \| Director [Member] \| Directors Plan [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	86,477	92,718	93,359
Share-based Payment Arrangement, Nonemployee [Member] \| Director [Member] \| Deferred Compensation, Share-based Payments [Member] \| Directors Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	23,659	17,170	17,289